(1) Field of the Invention
This invention relates to a device for controlling displacement of a variable displacement hydraulic pump adapted to be driven with a fixed displacement hydraulic pump by a common prime mover.
(2) Description of the Prior Art
In the prior art device for controlling displacement of a variable displacement hydraulic pump as driven by a prime mover, pressure in a discharge pipe of the variable displacement hydraulic pump is detected, and displacement is controlled to be increased or decreased in response to the pressure to render a drive torque of the variable displacement hydraulic pump constant and thereby to maintain an output of the prime mover at a constant level.
In the case that both the variable displacement hydraulic pump and a fixed hydraulic pump are driven by a common prime mover, as the output of the prime mover is fluctuated in association with fluctuation in load of the fixed displacement hydraulic pump, reduction in a rotational speed of the prime mover is detected as excess of load of the prime mover (overload of the fixed hydraulic pump) as well as displacement control of the variable displacement hydraulic pump, and according to such detection, the displacement of the variable displacement hydraulic pump is reduced to maintain the output of the prime mover at a constant level.
However, since such a control device as above is designed to detect reduction in engine rotational speed and input a detection signal to a displacement control system of the variable displacement hydraulic pump to control the displacement, there will be created delay of reponse time, and stable displacement control may not be provided.